Gray scale masks are useful in manufacturing various three dimensional mechanical, electrical and optical devices. For instance, gray scale masks are used to produce sophisticated geometrical structures or topographies necessary for creating mechanical structures, particular electrostatic field configurations, or optic structures. Gray scale masks may be used in micro-optic devices to produce well defined complex topologies used in refractive and diffractive optical elements. For example, gray scale masks may be used to produce small diffractive lenses, such as a blazed phase zone plate lens, for use in an optical head, as disclosed in U.S. patent application Ser. No. 08/833,608, entitled "Optical Head with a Diffractive Lens," by B. Block and A. Thornton, filed Apr. 11, 1997, which is herein incorporated by reference.
A gray scale mask is a two dimensional surface with varying optical transmissibility. The variations of the optical transmissibility represent three dimensional information, e.g., a height profile or depth pattern. The gray scale mask is used to transfer the three dimensional information to a resist layer on a substrate by photoexposure and development which leaves a modulated resist thickness. The three dimensional information now contained in the thickness modulated resist layer may subsequently be transferred into the substrate by known etching processes, thereby creating the desired depth pattern in the substrate. The resulting processed substrate then contains, as a physical contour, the three dimensional information that was originally represented by the variations of the optical transmissibility of the gray scale mask.
Gray scale masks are generally made out of a transparent substrate such as glass covered by an opaque, easily etched metallic layer. Chrome is often used because, among other reasons, it is easily deposited and etched. A gray scale can be created by the repetition of dots or pixels that appear as transparent holes in the chrome mask, as for example described in "One-step 3D Shaping Using a Gray-Tone Mask for Optical and Microelectronic Applications," Y. Oppliger et al., Microelectronic Engineering 23, p. 449 (1994); and in "One-Level Gray-Tone Lithography--Mask Data Preparation and Pattern Transfer," K. Reimer, et al., SPIE Vol. 2783, p.71, 1996, both of which are incorporated herein by reference. Conversely, a gray scale may be created by placing opaque pixels on a clear field. Until recently, there has been little need for extremely high resolution, expanded analog gray scale masks. An analog gray scale is expanded by making the pixels smaller while, if necessary, placing the pixels very close together, which is useful because it can more closely approximate a continuous transmissibility in the x-y plane. However, high resolution expanded analog gray scale masks are now in demand, for instance, to make a high resolution lens for a flying slider in an optical head as discussed in U.S. patent application Ser. No. 08/833,608.
The materials presently used in gray scale masks are responsible for the practical limitations on the resolution of the gray scale mask. For instance, with the use of direct electron beam (e-beam) writing, small geometric areas may be written upon, approximately 0.02 microns in line width. Chrome, however, which is commonly used in gray scale masks, does not permit the same resolution. Chrome is an isotropic material, and therefore when liquid etched, suffers from problems associated with isotropic etching, most notably undercutting. Undercutting is undesirable because the resolution of a gray scale mask and its accurate representation of three dimensional information are ultimately determined by the accuracy of the size, uniformity and variance of the pixels. These parameters are limited by the edge precision permitted by the material used in the gray scale mask. Because pixels, which are either transparent holes or opaque dots, are defined by their edges, an imprecise edge on a small pixel can drastically alter the size of the pixel. The undercutting process is difficult to control. Thus, there is limited accuracy of the size, uniformity and variance of the pixels in isotropic gray scale masking materials, such as chrome. Additionally, undercutting restricts the proximity of one pixel from another, which limits the extension of the analog gray scale. The rate of lateral etching in isotropic materials is approximately the same as the downward etching rate. Accordingly, in a chrome gray scale mask, the minimum distance between two pixels is approximately twice the thickness of the resist. This limitation is particularly detrimental in creating an extended analog gray scale mask.
Undercutting, which occurs in a wet etch process, may be avoided with dry chrome etching. The dry etch process, however, has other undesirable effects. Dry etching causes uncontrolled redeposition and thus, material may be unintentionally placed over pixels. Dry etching also damages the mask and substrate materials. Damage to the mask material may create unwanted holes or extra pixels in the mask, while damage to the substrate may interfere with the gray scale mask's transparency to light.
Gray scale masks made from chrome also suffer from light transmission problems, such as interference, standing waves, and scattered light. These problems are caused by the reflectivity of the chrome, and the surfaces created during the etching process, and may cause fluctuations in irradiation at different depths in the resist.
One response to the demand for higher resolution expanded analog gray scale masks is to use high energy beam sensitive glass (HEBS) as described in U.S. Pat. No. 5,078,771, issued to C. Wu on Jan. 7, 1992, which is incorporated herein by reference. Although HEBS glass can produce a high quality gray scale mask, to date, HEBS glass is not a standard commercially available glass, and therefore is difficult to obtain. The HEBS glass is an unusual formulation of a glass substrate and requires specialized treatment and is therefore expensive to manufacture Additionally, HEBS glass is unavailable to be optimized for specific requirements. The HEBS glass becomes opaque to deep UV radiation, giving a limit to its usable wavelength. Thus, HEBS glass is an unsatisfactory wide band ultraviolet transparent substrate for gray scale masks.